For me, Fakemon do enhance the experience of playing a Pokemon hack.
1) Originality. Less people have Fakemon because of their reputation and the additional work they need.

2) Difficulty. I know pretty much all of the Pokemon and their strengths and weaknesses. Adding Fakemon makes the hack more difficult without being ridiculous because you have to experiment first, you will not know their weaknesses right off the bat. You will also not know which Fakemon are powerful, so really, you will not know which Pokemon are ultimately powerful or weak, thus having your team composed of not just powerful Pokemon, but a more diverse group.

3) More suspense. You do not know the movepools of the Fakemon. You also do not know whether your Fakemon is going to evolve. When we do not know what is weak and powerful, we will be inclined to experiment and see which Fakemon are worth keeping. Also, hackers can be more creative with the story of their hack by adding a unique attribute to their main legendaries (if that is where the story is centered on.)

4) Wow factor. Since I would not know all of the Fakemon in a hack, whenever I see one that is ultra cool or ultra intimidating = wow.